The present invention relates to treatments useful in subterranean operations, and more particularly, to methods of modifying the surface stress-activated reactivity of proppant particulates used in subterranean operations.
In the production of hydrocarbons from a subterranean formation, the subterranean formation should be sufficiently conductive to permit the flow of desirable fluids to a well bore penetrating the formation. One type of treatment used in the art to increase the conductivity of a subterranean formation is hydraulic fracturing. Hydraulic fracturing operations generally involve pumping a treatment fluid (e.g., a fracturing fluid or a “pad fluid”) into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more pathways, or “fractures,” in the subterranean formation. These cracks generally increase the permeability of that portion of the formation. The fluid may comprise particulates, often referred to as “proppant particulates,” that are deposited in the resultant fractures. The proppant particulates are thought to help prevent the fractures from fully closing upon the release of the hydraulic pressure, forming conductive channels through which fluids may flow to a well bore.
One problem that may affect fluid conductivity in the formation after a fracturing treatment is the tendency for particulates (e.g., formation fines, proppant particulates, etc.) to flow back through the conductive channels in the subterranean formation, which can, for example, clog the conductive channels and/or damage the interior of the formation or equipment placed in the formation. One well-known technique to prevent these problems is to treat the associated portions of a subterranean formation with a hardenable resin to hopefully consolidate any loose particulates therein and to prevent their flow-back. Another technique used to prevent flow-back problems, commonly referred to as “gravel packing,” involves the placement of a gravel screen in the subterranean formation, which acts as a barrier that prevents particulates from flowing into the well bore.
The surfaces of proppant particulates generally comprise one or more minerals, which may react with other substances (e.g., water, minerals, treatment fluids, and the like) that reside in the subterranean formation in chemical reactions caused, at least in part, by conditions created by mechanical stresses on those minerals (e.g., fracturing of mineral surfaces, compaction of mineral particulates). These reactions are herein referred to as “stress-activated reactions” or “stress-activated reactivity.” One type of these stress-activated reactions is diageneous reactions. As used herein, the terms “diageneous reactions,” “diageneous reactivity,” and “diagenesis” are defined to include chemical and physical processes that move a portion of the mineral in a proppant particulate and/or convert a portion of the mineral in a proppant particulate into some other form in the presence of water. A mineral that has been so moved or converted is herein referred to as a “diageneous product.” Any proppant particulate comprising a mineral may be susceptible to these diageneous reactions, including natural silicate minerals (e.g., quartz), man-made silicates and glass materials, metal oxide minerals (both natural and man-made), and the like.
Two of the principle mechanisms that diagenesis reactions are thought to involve are pressure solution and precipitation processes. Where two water-wetted mineral surfaces are in contact with each other at a point under strain, the localized mineral solubility near that point increases, causing the minerals to dissolve. Minerals in solution may diffuse through the water film outside of the region where the mineral surfaces are in contact (e.g., the pore spaces of a proppant pack), where they may precipitate out of solution. The dissolution and precipitation of minerals in the course of these reactions may reduce the conductivity of the proppant pack by, inter alia, clogging the pore spaces in the proppant pack with mineral precipitate and/or collapsing the pore spaces by dissolving solid minerals in the “walls” of those pore spaces. In other instances, minerals on the surface of a proppant particulate also may exhibit a tendency to react with substances in formation fluids and/or treatment fluids that are in contact with the particulates, such as water, gelling agents (e.g., polysaccharides, biopolymers, etc.), and other substances commonly found in these fluids, whose molecules may become anchored to the mineral surface of the particulate. These types of reactivity may, inter alia, further decrease the conductivity of a subterranean formation through, inter alia, the obstruction of conductive fractures in the formation by any molecules that have become anchored to the proppant particulates resident within those fractures.
Another problem that may affect the conductivity of a formation arises as a result of the proppant particulates being under pressure while in contact with the surfaces of the subterranean formation, which can cause them to become embedded in the surfaces of the formation. This may damage the formation by forming “craters” therein. Among other things, these “craters” may be a source of damage to the formation and/or reduce the conductivity of the formation by reducing the width of fractures in which the proppant particulates reside. It is a known practice in the art to coat proppant particulates with resins and/or other substances to increase the ability of the proppant to withstand the pressure in a subterranean formation without becoming embedded in the surfaces of the formation.